1. Field of the Invention
This invention relates to a method for producing an optical waveguide part and an=optical waveguide part, more particularly a method for producing an optical waveguide part and an optical waveguide part suitable for a second harmonic generation (SHG) device utilizing a Quasi-Phase- Matching (QPM) system.
2. Related Art Statement
As a light source for a blue laser usable for an optical pickup, etc., a SHG device, utilizing a QPM system, is expected, which comprises an optical waveguide made of a ferroelectric single crystal such as a lithium niobate single crystal or a lithium tantalate single crystal. The optical waveguide has a periodically ferroelectric domain-inverted structure. The device may be widely used for optical disk memory, medicine, optochemical, a use of an optical pickup for various optical measurement.
In the SHG device, for obtaining a high conversion efficiency, the ferroelectric single crystal is required to have a ferroelectric domain-inverted structure with deep domains.
As a method for forming deep ferroelectric domain-inverted structures, a ferroelectric domain-inverted pattern is formed in a given ferroelectric single crystalline substrate, a ferroelectric single crystalline film is epitaxially grown on the substrate, and the pattern is transcribed and formed in the film.
For example, JP A 5-173213, JP A 6-67233, and JP A 6-160927 disclose the following method. That is, a ferroelectric single crystalline substrate 1, made of Z-cut lithium niobate single crystal, lithium tantalate or the like, is prepared. Then, as shown in FIG. 1, a rectangular electrode 2 is formed on one main surface of the substrate 1 and a planar electrode 3 is formed on the other main surface, and a given voltage V1 is supplied between the electrodes 2 and 3. Thereafter, as shown in FIG. 2, a ferroelectric single crystalline film 6 is formed on the substrate 1 by a liquid epitaxial method and a ferroelectric domain-inverted pattern 5 of the substrate 1 is transcribed into the film to form a ferroelectric domain-inverted structure therein.
xe2x80x9cJ. Appl. Phys. Lett.xe2x80x9d No. 65, 1994, p2154-2155 discloses a method in which a periodically ferroelectric domain-inverted pattern is formed in a ferroelectric single crystalline substrate made of a Z-cut lithium niobate single crystal by a Ti diffusion method, a ferroelectric crystalline film made of a lithium niobate single crystal is epitaxially grown on the substrate, and the pattern is transcribed into the film to form a ferroelectric domain-inverted structure therein.
In each of the above methods, however, a Z-cut face of a ferroelectric single crystal is employed as the ferroelectric single crystalline substrate. Thus, as shown in FIGS. 2(a) and 2(b), the ferroelectric domain-inverted structure, transcribed into the ferroelectric single crystalline film, which is formed by the liquid phase epitaxial method, has only ferroelectric domain-inverted structures 7 and 9 having ferroelectric domain-inverted patterns 8 and 10 with a period of about 1 xcexcm of which width is increased or decreased with a growth of the ferroelectric single crystalline film 6. Consequently, it is difficult to form a ferroelectric domain-inverted structure having a rectangular pattern with deep domains.
xe2x80x9cJ. Appl. Phys. Lett.xe2x80x9d No. 65, 1994, p2154-2155 also describes the above fact.
It is an object of the present invention to provide an optical waveguide part with a ferroelectric domain-inverted structure having a deeply rectangular pattern in a ferroelectric single crystal and a method for producing the same.
This invention relates to a method for producing an optical waveguide part, including the steps of preparing a ferroelectric single crystalline substrate having a polarization-axis substantially parallel to a main surface thereof and having a given ferroelectric domain-inverted pattern, and epitaxially growing a ferroelectric single crystalline film on the ferroelectric single crystalline substrate. The ferroelectric domain-inverted pattern is thereby transcribed from the substrate into the ferroelectric single crystalline film to form a ferroelectric domain-inverted structure therein.
This invention also relates to an optical waveguide part, including a ferroelectric single crystalline substrate having a given ferroelectric domain-inverted pattern and a ferroelectric single crystalline film having a ferroelectric domain-inverted structure composed of a ferroelectric domain-inverted pattern with substantially the same shape, as viewed in a vertical direction to the optical waveguide part, as the one of the ferroelectric single crystalline substrate.
In the conventional method, as mentioned above, in which the ferroelectric domain-inverted pattern is transcribed and formed in the ferroelectric single crystalline film, a polarization-axis of the film is parallel to a Z-axis thereof, so that a relatively deep ferroelectric domain-inverted is likely to be formed along the Z-axis. Thus, it is conventionally known to use a Z-cut ferroelectric single crystalline substrate. However, there has been no detailed investigation concerning other cut-faces of ferroelectric single crystalline substrates.
The inventors discovered the following facts. That is, a ferroelectric single crystalline substrate 11, made of an X-cut lithium niobate single crystal, is prepared and an comb-shaped electrode 12 and a uniform electrode 13 are formed on the substrate 11. Then, a voltage V2 parallel to a main surface of the substrate 11 is supplied to the substrate 11 to form a ferroelectric domain-inverted pattern 15. Next, a ferroelectric single crystalline film 16, made of a lithium niobate, is formed on the substrate 11 by the liquid phase epitaxial method and thereby a deeply rectangular ferroelectric domain-inverted pattern 18 is transcribed and formed from the ferroelectric domain-inverted pattern 15. The present invention is based on the above finding.
According to the present invention, there can be provided an optical waveguide part having a ferroelectric domain-inverted structure 17 with a rectangular pattern including deep domains of which each has parallel ferroelectric domain-boundaries. Thus, a QPM-SHG device having a high conversional efficiency can be obtained.
As discussed above, when employing a ferroelectric single crystalline substrate having a ferroelectric polarization-axis substantially parallel to a main surface of the substrate, a ferroelectric domain-inverted structure having a rectangular ferroelectric domain-inverted pattern is formed in a ferroelectric single crystalline film formed on the main surface of the substrate. The inventors could find no clear reason for this, but is presumed to be the following.
That is, it is presumed that since an interface energy of a ferroeleetric domain-boundary (between an inverted part and a non-inverted part) in forming the ferroelectric domain-inverted pattern vertical to a Z-axis is smaller than one in forming it parallel to the Z-axis, the shape of the inverted pattern is unlikely to degrade.